The present invention relates to an apparatus for determining the distance between two surfaces which are in communication with upper and lower mold halves in a press, particularly mold halves for shaping fiber-reinforced molding compound, with the mold halves being adapted to one another in such a manner that they assume an end position corresponding to the desired thickness of the molded object when the space between the surfaces communicating with the mold halves has attained a limit value.
When a molded object is made by compressing fiber-reinforced molding compound, care must be taken that the pressing force acts exclusively on the molding compound. The pressing force should not be absorbed, even in part, by the lands of the mold halves. The lands are adapted with respect to their position and configuration so that the molded object to be produced has the desired thickness when there is a small predetermined distance between the lands--for example in the order of magnitude of 0.2 mm.
In the press for processing fiber-reinforced molding compound involved here, it may be difficult to conduct the distance measurement for achieving the desired thickness of the molded object. The press may easily reach temperatures up to 200.degree. C. Furthermore soiling occurs during the molding process, so cleaning is needed from time to time. Both facts make the use of reliably operating measuring devices at least questionable.
A prior art example of the use of distance-measuring transducers during compression molding will now be discussed with reference to FIG. 2. In FIG. 2, a press 20 includes an upper platen 22 which is raised or lowered by a ram 24. A heating table 26 is affixed to a lower platen 28. An upper mold half 1 is mounted on upper platen 22 and a lower mold half 2 is mounted on heating table 26. It will be apparent that the mold halves 1 and 2 cooperate to form a mold which, in the closed position as shown, defines a mold cavity. Adjacent the mold cavity, mold half 1 is provided with lands 1a and mold half 2 is provided with lands 2a.
With continuing reference to FIG. 2, the platens 22 and 28 are rectangular, with the plains of the respective rectangles being perpendicular to the plane of the drawing. Four linear variable differential transformers (LVDTs) 30 (only two of which are shown) are mounted on lower platen 28 adjacent the corners of the rectangle to provide distance-measuring transducers. The LVDTs 30 come into contact with upper platen 22 as RAM 24 is being lowered, and during further closing movement of the mold the LVDTs 30 generate signals corresponding to the distance between platens 22 and 28 at the four corners of the rectangle. At least one hydraulic cylinder 32 is provided adjacent every LVDT 30. The hydraulic cylinders 32 selectively generate upward forces adjacent the corners of the rectangle to resist the downward force imparted by RAM 24 as the mold is closed.
During a compression molding procedure, the ram 24 is raised to open the mold. A measured charge of molding compound 34 is placed at the center of lower mold half 2. A typical molding compound would include about one third polyester resin, about one third to two thirds glass fiber, and the rest a filler such as calcium carbonate. Ram 24 is then actuated to begin closing the mold. As the mold-closing process progresses the LVDTs 30 generates signals which are supplied to an electrical controller (not illustrated). The controller selectively actuates hydraulic cylinders 32 to ensure that the mold halves 1 and 2 remain parallel and to control the relative velocity between mold halves 1 and 2. The relative velocity is reduced to zero when LVDTs 30 signal that the distance between mold halves 1 and 2 has reached th desired value, and thus that the molded object has attained the desired thickness.
Further details about press 20 and its use during a compression molding procedure can be found in U.S. Pat. No. 4,076,780, the disclosure of which is incorporated herein by reference.